It is known that lactoferrin is a natural iron-binding protein occurring in vivo, e.g. in lacrima, saliva, peripheral blood, milk and the like, and that it exhibits antimicrobial activity against various harmful microorganisms belonging to the genera of Escherichia, Candida, Closridium, and the like (Journal of Pediatrics, Vol. 94, Page 1, 1979). It is also known that lactoferrin exhibits antimicrobial activity, in a concentration of 0.5-80 mg/ml, against microorganisms belonging to the genera of Staphylococcus and Enterococcus (Nonnecke, B. J. and Smith, K. L.: Journal of Dairy Science, Vol. 67, page 606, 1984).
On the other hand, a number of inventions are known for peptides having antimicrobial activity against various microorganisms. Some examples of such peptides are: phosphono-tripeptide (Japanese Unexamined Patent Application Gazette No. 57(1982)-106689), phosphono dipeptide derivatives (Japanese Unexamined Patent Application Gazette No. 58(1983)-18594), and cyclic peptide derivatives (Japanese Unexamined Patent Application Gazette No. 58(1983)-213744) which are effective against Gram positive and Gram negative bacteria; peptides having antimicrobial and antiviral activities (Japanese Unexamined Patent. Application Gazette No. 59(1984)-51247); polypeptides effective against yeast (Japanese Unexamined Patent Application Gazette No. 60(1985)-130599); glycopeptides derivatives effective against Gram positive bacteria (Japanese Unexamined Patent Application Gazette Nos. 60(1985)-172998, 61(1986)-251699, 63(1988)-44598); oligopeptides effective against Gram positive bacteria (Japanese Unexamined Patent Application Gazette No. 62(1987)-22798); peptide antibiotics (Japanese Unexamined Patent Application Gazette Nos. 62(1987)-51697, 63(1988)-17897); antimicrobial peptides extracted from blood cells of Tachypleus tridentalus from North America (Japanese Unexamined Patent Application Gazette No. Heisei 2(1990)-53799); antimicrobial peptides isolated from hemolymph of bees (Japanese Unexamined Patent Application (via PCT root) Gazette No. Heisei 2(1990)-500084), and the like.
The inventors of this invention contemplated to isolation of useful substances, which do not have undesirable side effects (e.g. antigenicity) and which have heat-resistance as well as potent antimicrobial activity, from nature at a reasonable cost, and found the fact that hydrolysates of lactoferrin obtainable by acid or enzyme hydrolysis of mammalian lactoferrin, apo-lactoferrin, and/or metal chelated lactoferrin (hereinafter they are referred to as lactoferrins) have more potent heat-resistance and antimicrobial activity than unhydrolyzed lactoferrins, for which a patent application has been filed (Japanese Patent Application No. Heisei 3(1991)-171736).
Furthermore, the inventors of this invention previously found a number of peptides, originated from the lactoferrins, which do not have side effects (e.g. antigenicity), and which have heat-resistance as well as a potent antimicrobial activity, e.g. antimicrobial peptides having 20 amino acid residues (Japanese Patent Application No. Heisei 3(1991)-186260), antimicrobial peptides having 11 amino acid residues (Japanese Patent Application No. Heisei 3(1991)-48196), antimicrobial peptides having 6 amino acid residues (Japanese Patent Application No. Heisei 3(1991)-94492), antimicrobial peptides having 5 amino acid residues (Japanese Patent Application No. Heisei 39(1991)-94493), and antimicrobial peptides having 3-6 amino acid residues (Japanese Patent Application No. Heisei 3(1991)-94494), for which patent applications have been filed.
Heretofore, various studies have been made to potentiate the antimicrobial activity of lactoferrin, and IgA and glycopeptides are known as the auxiliary agents for potentiating such a physiological activity. There are many reports in this respect, for example, a method for the potentiation of the antimicrobial activity of lactoferrin by coexistence of lysozyme therewith (Japanese Unexamined Patent Application Gazette No. 62(1987)-249931), a method for potentiation of antimicrobial activity of lactoferrin by the coexistence of secretory IgA therewith (Stephens, S. et al.: Immunology; Vol. 41, Page 597, 1980) and so on. Furthermore, Spick et al. report that lactoferrin has an activity of inhibiting bacteria from adhering onto mucous membrane, and that this activity is potentiated by the coexistence of lysozyme or glycopeptides (Edit. by William, A. F. and Baum, J. D.: "Human Milk Banking", Nestle Nutrition Workshop Series, Vol. 5, Page 133, Pub. by Raven Press Books, Ltd.).
The efficacy of the combined use of lactoferrin and antibiotics has been also studied, and cephem antibiotics (Miyazaki, S. et al.: Chemotherapy, Vol. 39, Page 829, 1991), .beta.-lactum antibiotics (Japanese Unexamined Patent Application Gazette No. Heisei 1-319463), and the like are known as the antibiotics which may potentiate antimicrobial activity upon the combined use with lactoferrin.
However, there have been no studies about the efficacy of the combined use of antimicrobial peptides derived from lactoferrins and specific compounds and/or antibiotics, consequently there have been no antimicrobial agents containing such substances as their effective ingredients. Furthermore, there has been no attempt to treat various matters such as foods, medicines and the like with such an agent.